Geographical Research

The study of spatial and temporal Changes in climate parameters، is one the most important issues in the world''s climate research. Study of changes in cloud cover as one of the most important climate parameters is important، because clouds are in top of hydrologic cycle and changes in them make changes in spatial and temporal variations in other climate parameters. In this study، the spatial variations in cloud cover in country by using monthly data of 90 synoptic stations (in 03، 09، 15 Greenwich)، and cluster analysis on 18*18 kilometers cell size were done. Also the temporal variations and change-point in cloud cover time series was evaluated by using Mann- Kendall and Petitt'' tests. The results showed that the clustering method can classify the amount of cloud cover in the country. This method showed five separate area for the annual amount of cloud cover including South Coast region of the Caspian Sea، Azerbaijan and Alborz regions، western Zagros and South slopes of Alborz، south-western and northern Central Desert region and south and central were presented. These areas have a good consistence with old finding about the synoptic patterns of air masses and cyclones traces in Iran. The results of temporal analysis also showed that the trend in the amount of cloud cover in most stations of areas 3، 4 and 5 (northern half of the country) is statistically significant. These significant trends are negative for all station، in other words the amount cloud is reduced. The results also showed that the sudden change in the series of cloud cover values in most of these stations occurred in 1990s.